Fix dependency installation instructions

Technically speaking, `yum` is a RHEL thing, not an RPM thing :)

There's a number of other RPM-based package managers for different non-RHEL RPM distributions, like `zypper` for openSUSE and `apt-rpm` for PCLinuxOS. I believe the command for PCLinuxOS is the same as for a Debian-based distribution, but I'm not running PCLinuxOS myself and I'm not sure whether wmctrl is in the default repositories, so I haven't changed/added that.